Improvement in the performance of information processing apparatuses, such as apparatuses or servers for communication infrastructure, necessarily involves a high data rate for signal transmission/reception in and out of the apparatuses. In a communication system that performs data signal transmission/reception, a data acquisition circuit (e.g., a flip-flop circuit) in a signal recovery circuit acquires a transmitted data signal according to a received signal recovery clock in order to perform the signal recovery. As a received signal recovery clock, a clock recovered from a received data signal by a clock and data recovery (CDR) circuit is generally used. The received signal recovery clock may also be generated by adjusting the phase of a clock generated in the signal recovery circuit to follow the intermediate phase of a transition edge of the received data signal. In the following, a case where a clock recovered by the CDR circuit for use as a received signal recovery clock will be described by way of example.
In the signal recovery circuit, the data acquisition circuit (e.g., a flip-flop circuit) acquires a received data signal according to a received signal recovery clock recovered by the CDR circuit in order to perform a signal recovery. Since the uttering of phase direction is eliminated by performing the acquisition according to the received signal recovery clock, this operation is called retiming as well. In retiming, sampling (acquisition) is performed according to the received signal recovery clock rising at the center of a transition edge of the received data signal, i.e., at a phase differed 180 degrees relative to the transition edge to determine whether the received data is 0 or 1.
In an optical communication system, an optical signal is generated by directly modulating a laser diode (LD) provided in a transmitter by a transmitted data signal. The optical signal is transmitted to a receiver via an optical cable, and a photodetector (PD) in the receiver converts the optical signal into a received data signal in the form of an electric signal. A signal recovery circuit in the receiver recovers a received signal recovery clock signal from the received data signal and performs the retiming on the received data signal according to the recovered received signal recovery clock signal.
The optical signal generated by directly modulating the LD asymmetrically rises and falls due to the characteristic nature of the LD. Specifically, the optical signal transitions faster when a transmitted data signal rises from 0 to 1 than when the transmitted data signal falls from 1 to 0. In other words, the optical signal transitions faster from a low strength level to a high strength level and transitions slower from a high strength level to a low strength level. Thus, the received signal also transitions faster when rising from 0 to 1 and transitions slower when falling from 1 to 0.
However, the signal recovery circuit of the optical communication system as above has performed retiming by a received signal recovery clock that rises at a phase differed 180 degrees from the transition edge of the received data signal regardless of the received data.
As described above, although rises and falls in the received data signal are asymmetric, retiming had been performed by the received signal recovery clock that rises at a phase differed 180 degrees from the transition edge of the received data signal. Thus, the transition edge of the received signal recovery clock interferes with the rising and falling of the received data signal, thereby causing an error in the received data.
In the above, a case where a received data signal rises fast and falls slow is described by way of example. The same problem as above also occurs in a reverse case, i.e., where the received data signal rises slow and falls fast.
The following are reference documents.                [Document 1] Japanese Laid-Open Patent Publication No. 2006-253808,        [Document 2] Japanese Laid-Open Patent Publication No. 2006-041818,        [Document 3] E. Haglund, et al,“25 Gbit/s transmission over 500 m multimode fiber using 850 nm VCSEL with integrated mode filter”, ELECTRONICS LETTERS 26 Apr. 2012, Vol. 49, No. 9.        